Not applicable.
This invention relates to a proximity detector including a Hall-voltage peak-to-peak percentage threshold detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and trailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles, and more particularly relates to such a Hall sensor with detection thresholds that adapt to the peak to peak amplitude of the Hall voltage.
The term xe2x80x9cmagneticxe2x80x9d as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field.
In the patent U.S. Pat. No. 5,442,283, issued Aug. 15, 1995 there is described a proximity detector including a Hall-voltage slope-activated, or peak-referenced detector capable of detecting the rising and falling edges of a gear tooth. The detector includes a circuit for tracking a slope of a Hall voltage and briefly holding the ensuing peak voltage before producing a pulse signal indicating the onset of the following Hall-voltage slope of opposite direction. The Hall voltage holding circuit includes a capacitor and circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides a pulse output signal. The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear tooth speed at which accurate detection is possible.
The changes in the ambient magnetic field and corresponding changes in the transducer voltage caused by the passing of magnetic articles tend to vary. Most such proximity detectors of the prior art produce a high binary output voltage indicating proximity of a passing article, and produce a low binary voltage when the article recedes from the detector.
The transition in detector output voltage from low to high typically is triggered by a comparator that determines when the transducer voltage rises to equal a fixed internal threshold voltage reference, or in the case of the above described slope-activated, or peak-referenced detector, determines when a transducer voltage peak has just occurred and the signal voltage drops a predetermined incremental voltage from the peak value.
These prior art proximity detectors, having fixed threshold voltages, produce a low to high (or high to low) output voltage that corresponds to different locations in the transducer voltage waveform when there are changes in the amplitude of the transducer voltage.
The sources of such changes in transducer voltage amplitude are many. For example, gear teeth (articles) may have different ferro-magnetic properties from tooth to tooth and undulating changes in the spacings (air gap) gear teeth to transducer caused by eccentricity of the gear. Also, changes in temperature can cause changes in air gap dimensions and in the sensitivity of the transducer and transducer-voltage amplifier. Furthermore, the magnetic-field to voltage transducer in a proximity detector typically includes an internal DC offset voltage that varies with mechanical stresses and temperature.
Such changes in the transducer voltage therefore cause shifts in the timing of proximity detection relative to the actual distances of article approach and receding at which these transducer voltages exceed or fall below the fixed thresholds. This results in loss of accuracy in proximity detection that has become less and less tolerable especially when employed for detection of the rotational position of a gear by sensing the proximity of the gear teeth.
It is an object of this invention to provide a proximity detector that generates a binary output voltage wherein the transitions accurately correspond to a definite point of approach and a definite point of receding of a passing magnetic article.
It is a further object of this invention to provide a magnetic article proximity detector that periodically determines when the amplitude or offset of the magnetic-field-to-voltage transducer voltage has changed significantly, and adjusts the detection threshold as needed to be essentially a predetermined constant percentage of the peak to peak value of a changing detector-transducer-voltage amplitude.
The invention relates to a method for detecting passing magnetic articles which includes an initial step of sensing an ambient magnetic field and generating a voltage, Vsig, proportional to the magnetic field. A threshold voltage is generated as a percentage of the peak-to-peak voltage of Vsig. The method further includes the step of generating a detector output voltage that becomes a first binary level when Vsig rises to exceed the threshold voltage and a second binary level when Vsig falls to below the threshold voltage. More particularly, a PDAC voltage is generated as a function of the positive peak values of Vsig and an NDAC voltage is generated as a function of the negative peak values of Vsig. The threshold voltage is updated by a predetermined amount upon each transition of the detector output voltage and is further updated to track the positive and negative peaks of the Vsig voltage.
With this arrangement, a relatively simple and robust circuit and technique are provided for updating the PDAC and NDAC voltages to ensure that the threshold voltage remains, within a predetermined tolerance, a percentage of the peak-to-peak Vsig voltage. This method further provides additional hysteresis which serves to reduce the susceptibiltiy of the circuit to noise on the Vsig signal and which is introduced at a time when the switch points defining transitions of the detector output voltage are not affected. Further, the method has a relatively fast response time, since complex threshold voltage updating decisions are eliminated by updating the threshold voltage by a predetermined amount after transitions of the detector output voltage. Additionally, the simplicity of the threshold voltage updating technique results in simplified circuit testing and thus, a reduction in manufacturing time and cost.
The threshold voltage is at a first level corresponding to a first percentage of the peak-to-peak Vsig voltage when Vsig exceeds the threshold voltage and is at a second level corresponding to a second percentage of the peak-to-peak Vsig voltage when Vsig is less than the threshold voltage. With this arrangement, the threshold voltage is provided with hysteresis. More particularly, the first level of the threshold voltage is a first percentage of the voltage PDACxe2x88x92NDAC and the second level of the threshold voltage is a second percentage of the voltage PDACxe2x88x92NDAC.
The threshold voltage updating step includes decreasing the PDAC voltage by the predetermined amount upon transitions of the detector output signal from one of the binary levels to the other and increasing the NDAC voltage by the predetermined amount upon opposite transitions of the detector output signal. The threshold voltage updating step further includes permitting the PDAC voltage to track the positive peaks of the Vsig signal and permitting the NDAC voltage to track the negative peaks of the Vsig voltage.
Also described is a magnetic article detector comprising a magnetic field sensor providing a voltage output signal, Vsig, proportional to the magnetic field, a threshold voltage generator operative to generate a threshold voltage that is a percentage of the peak-to-peak voltage of Vsig and a comparator comparing Vsig to the threshold voltage to generate a detector output voltage. The detector output voltage becomes one binary level when Vsig rises to exceed the threshold voltage and another binary level when Vsig falls to below the threshold voltage. The threshold voltage is updated by a predetermined amount upon each transition of the detector output voltage and is further updated to track the positive and negative peaks of the Vsig voltage. A hysteresis circuit provides the threshold voltage at a first level that is a first percentage of the peak-to-peak voltage of Vsig when Vsig exceeds the threshold voltage and at a second level that is a second percentage of the peak-to-peak voltage of Vsig when Vsig is below the threshold voltage.
The threshold voltage generator includes a PDAC voltage generator, an NDAC voltage generator, and a circuit coupled between the PDAC voltage and the NDAC voltage for providing the threshold voltage as a percentage of the difference between the PDAC voltage and the NDAC voltage. The PDAC voltage generator includes a first counter having an output at which a first count signal is provided and a first digital to analog converter having an input coupled to the output of the first counter and an output at which the PDAC voltage is provided. The first counter counts up causing the PDAC voltage to increase when the Vsig voltage is greater than the PDAC voltage, thereby causing the PDAC voltage to track the positive peaks of the Vsig voltage. The first counter counts down for a predetermined duration upon transitions of the detector output voltage of a first polarity, thereby decreasing the PDAC voltage by a predetermined amount. Similarly, the NDAC voltage generator includes a second counter having an output at which a second count signal is provided and a second digital to analog converter having an input coupled to the output of the second counter and an output at which the NDAC voltage is provided. The second counter counts down causing the NDAC voltage to track the negative peaks of the Vsig voltage and counts up for a predetermined duration upon transitions of the detector output voltage of a second polarity, thereby increasing the NDAC voltage by a predetermined amount.